The present invention pertains generally to medical catheters and methods for manufacturing medical catheters. More particularly, the present invention pertains to medical catheters having improved shape retention. The present invention is particularly, but not exclusively, useful as an angioplasty catheter for movement through vessels wherein turns of up to 120 degrees are required.
Intravascular procedures are commonly utilized to treat a stenosis within a vessel or artery of a human. One procedure used to treat a stenosis is commonly referred to as angioplasty. During an angioplasty procedure, a guidewire is first positioned in the vessel to establish a mechanical pathway to the stenosis. Next, a balloon catheter is placed over the guidewire and pushed through the vasculature until the balloon is adjacent the stenosis. Finally, the balloon is inflated to compress the stenosis and thereby dilate the lumen of the vessel.
The human vasculature is curved, branched and contains vessels having relatively small inner diameters. As a result thereof, the doctor or physician often needs to maneuver and twist the catheter to move the catheter through the body vessel. In some circumstances, for example near the aorta, the catheter must be capable of bending over 120 degrees as it is advanced through the curved vessel. To do this, the shaft of the catheter must have good strength and stiffness to withstand the axial and torsional forces which occur as the catheter is pushed and steered through the vasculature. Additionally, however, the catheter shaft must be sufficiently flexible to allow the catheter to track the guidewire.
The tradeoff between stiffness and flexibility may be partially overcome by using a two-part catheter shaft. Specifically, the distal portion of the shaft, where flexibility is required for adequate tracking, can be made of a flexible material such as plastic. On the other hand, in the proximal portion of the shaft, where more strength and stiffness are required for adequate pushability, the shaft can be made from a metallic material such as stainless steel. Unfortunately, for such a two-part construction, the area of the catheter shaft near the joint between the flexible portion and the stiff portion is generally subject to kinking. In particular, when the catheter shaft is made to bend through angles of up to 120 degrees, the area near the joint absorbs nearly all the stress of the bend. The result can be a sharp bend in which permanent deformation occurs. This permanent deformation or kink does not recover as the joint area subsequently passes into straighter vessel paths. Rather, the kink interferes with and limits the subsequent movement of the catheter throughout the vasculature.
Certain alloys, called shape-memory alloys, are known for their ability to recover large strains (up to approximately 8 percent). As is well known, the crystal structure of alloys can be manipulated by thermal treatments and other processes to alter the microstructure of the alloy from one crystal structure to another. Each crystal structure is known as a phase, such as an austenite phase or a martensite phase, and the change from one phase to another is termed a phase transformation. To use a traditional shape-memory alloy, a part is initially shaped from the alloy at a first temperature, above the phase transformation temperature. Next, the shaped part can be cooled to a second temperature, below the phase transformation temperature, thus inducing a phase transformation such as an austenite to martensite phase transformation. At the lower temperature, while the alloy is in the martensite phase, a stress can be applied to deform the part to strains of up to approximately 8 percent. Upon release of the applied stress, the 8 percent strain will remain. Next, the deformed part can be heated back above the phase transformation temperature, thereby transforming the alloy back to the austenite phase. During this last phase transformation, the strain will be recovered, and the original (unstrained) shape of the part will return.
Some shape memory alloys will isothermally transform from the austenite phase to the martensite phase in response to an applied stress. These alloys are called stress-induced martensite (SIM) alloys. For example, at a temperature slightly above the austenite to martensite phase transformation temperature, the SIM alloy can be isothermally deformed (up to 8 percent) causing the alloy to transform from the austenite to the martensite phase. In the absence of the phase transformation, a strain of 8 percent could not be recovered. In the SIM alloy, when the stress is removed, the alloy will return to the austenite phase and strain will be recovered. Importantly, the strain and recovery process in SIM alloys can occur isothermally. The ability of SIM alloys to recover large strains isothermally through the phase transformation process is termed superelasticity.
Importantly for the present invention, some SIM alloys are known in the art, such as some nickel-titanium alloys, that have an austenite to martensite phase transformation temperature slightly below the human body temperature. Consequently, in light of the above discussion, these alloys are superelastic when positioned inside a body vessel. When a part made from these alloys is inserted into the body and subsequently placed under stress creating deformations of up to 8 percent, these deformations or strains can be recovered when the stress is removed. For example, a catheter shaft made of a SIM alloy may bend while negotiating a 120 degree curve in a vessel. During the bend, strains of up to 8 percent may occur near the outer radius of the shaft. As the deformed portion of the shaft is advanced from the curved portion of the vessel to a straighter vessel portion, the stress from the bend will recover, and the deformed portion of the shaft will return to its original shape.
In light of the above, it is an object of the present invention to provide a catheter and a method of manufacturing a catheter having good pushability and trackability in the body vessel. Another object of the present invention is to provide a catheter and a method of manufacturing a catheter that can traverse a path having a 120 degree bend within the vasculature of a patient without kinking. Still another object of the present invention is to provide a catheter and a method of manufacturing a catheter having good flexibility, durability, and torsional strength characteristics. Another object of the present invention is to provide a joint that creates a gradual transition between a stiff proximal shaft and a flexible distal tube. Yet another object of the present invention is to provide a catheter which is relatively simple to manufacture, is easy to use, and is comparatively cost effective.
The present invention is directed to a catheter and a method for manufacturing a catheter for use in a body vessel. For the present invention, the catheter includes a relatively stiff proximal shaft that is shaped as a tube formed with a lumen and having a proximal end and a distal end. Further, the catheter includes a relatively flexible distal tube formed with a lumen and having a proximal end and a distal end. Each tube has an inner wall and an outer wall. An inflatable balloon may be attached to the outer wall of the distal tube near the distal end. The proximal end of the distal tube is attached to the distal end of the proximal shaft at a transitional joint which provides a gradual transition in flexibility over the length of the joint between the stiff proximal shaft and the flexible distal tube.
Importantly, the transitional joint for the catheter of the present invention includes an extension made of a material having superelastic properties when positioned inside the body vessel. Functionally, the extension allows for the gradual transition in flexibility required in the joint section. One end of the extension is attached to the distal end of the proximal shaft. The other end of the extension projects into the lumen of the distal tube. The extension may be formed as a ribbon, core wire or may constitute a tapered section of the proximal shaft. Using these component elements, at least three embodiments for a transitional joint are contemplated for the present invention for interconnecting the distal tube, the extension, and the proximal shaft.
In a first embodiment, in addition to the distal tube, extension, and proximal shaft, the transitional joint includes an extension tube and an insert. Preferably, the extension tube and insert are made from a material having superelastic properties when positioned inside the body vessel. In this embodiment, one end of the insert is disposed within the lumen of the proximal shaft and is affixed to the inner wall of the proximal shaft. The second end of the insert is disposed within the lumen of the extension tube and is affixed to the inner wall of the extension tube. Further, the extension is affixed to the outer wall of the extension tube, thereby becoming attached to the proximal shaft. The extension then projects into the lumen of the distal tube. In this embodiment, the inner wall near the proximal end of the distal tube is affixed to the outer wall of the extension tube, thereby attaching the distal tube to the proximal shaft.
In another embodiment of the transitional joint for attaching both the distal tube and the extension to the proximal shaft, a tapered section formed integrally with the proximal shaft and extending from the distal end of the proximal shaft constitutes the extension. In this embodiment, the tapered section (extension) projects into the lumen of the distal tube. The inner wall near the proximal end of the distal tube is bonded to the outer wall of the proximal shaft near its distal end. Preferably, in this embodiment, both the proximal shaft and extension are made from a material having superelastic properties when positioned inside the body vessel.
In yet another embodiment of the transitional joint for attaching both the distal tube and the extension to the proximal shaft, the catheter includes a coil spring made from a material having superelastic properties when positioned inside the body vessel. The coil spring has an inner wall and an outer wall, and is formed with a lumen. Further, the coil spring has a proximal end and a distal end. In this embodiment, the distal end of the proximal shaft is deformed or collapsed. This deformation results in a portion of the outer wall of the proximal shaft at the distal end having a concave surface. The inner wall of the coil spring near the proximal end is affixed to the outer wall of the proximal shaft adjacent and proximal to the concave surface. The outer wall near the distal end of the coil spring is affixed to the inner wall of the distal tube thereby attaching the distal tube to the proximal shaft. In this embodiment, the extension is preferably a core wire. One end of the core wire is affixed to the concave surface of the proximal shaft and the other end of the core wire projects through the lumen of the coil spring and into the lumen of the distal tube.